1. Field of the Invention
The present invention relates to an electrical steel sheet, for a low noise transformer excellent in magnetostriction property, used for an iron core of a transformer and the like.
2. Description of the Related Art
In magnetic materials widely used for electrical and electronic apparatuses the lengths change when a magnetic field is applied (this is referred to as xe2x80x9cmagnetostrictionxe2x80x9d) to cause of transformer noise and the extent thereof has become an important evaluation item in quality control. In recent years, regulation of noise generated by electrical apparatuses is becoming stricter with an increased demand for a better living environment. Therefore, studies for lowering noise, by decreasing magnetostriction, are being carried out.
Among magnetic materials, for grain-oriented electrical steel sheets used for iron cores of transformers, a means for decreasing magnetostriction by reducing closure domains has been applied. The term xe2x80x9cclosure domainsxe2x80x9d as cited here means domains having magnetization oriented in the direction perpendicular to the direction of an applied magnetic field. Magnetostriction is caused when this magnetization is turned to a direction parallel to the magnetic field by an applied magnetic field. Accordingly, the fewer the closure domains, the smaller the magnetostriction. The following methods are known as means for decreasing magnetostriction:
{circle around (1)} a method for avoiding the formation of closure domains, that cause shape changes by magnetization rotation, by means of aligning the  less than 001 greater than  direction of crystal grains with the rolling direction. (T. Nozawa et al, xe2x80x9cRelationship between Total Losses under Tensile Stress in 3 Percent Sixe2x80x94Fe Single Crystals and Their Orientations near (110) [001],xe2x80x9d IEEE Trans. on Mag., Vol. MAG-14, No.4, 1978),
{circle around (2)} a method for eliminating closure domains by releasing plastic strains (Japanese Unexamined Patent Publication No. H7-305115, [Development of Revolutionary Oriented Silicon Steel Sheet, ORIENTCORE HI-B]: OHM 1972. 2), and
{circle around (3)} a method for eliminating closure domains by applying a tension film to a steel sheet (T. Nozawa et al, xe2x80x9cRelationship between Total Losses under Tensile Stress in 3 Percent Sixe2x80x94Fe Single Crystals and Their Orientations near (110) [001],xe2x80x9dIEEE Trans. on Mag., Vol. MAG-14, No.4, 1978).
These three methods have decreased magnetostriction and have contributed to lowering the noise of electrical apparatuses.
When a transformer is fabricated from a grain-oriented electrical steel sheet and magnetized, various modes of vibrations are generated in the structure and the oscillation frequencies of harmonics are also generated. In particular, a fundamental frequency of magnetization (for example, 100 Hz in the case of an exciting current of 50 Hz in frequency) and frequencies obtained by multiplying the fundamental frequency by integral numbers (for example, 200, 300, 400 Hzxe2x80x94in the case of an exciting current of 50 Hz in frequency) have especially high intensity among transformer noises. Among these frequencies, relatively low frequency components vibrate an iron core proper directly, while high frequency components resonate the auxiliary units of a transformer, such as a tank, a chiller, a conservator and the like. However, since the vibrations decrease heir intensity exponentially and become less influential at higher frequencies, technologies for reducing the low-frequency components have mainly been tackled so far.
However, demands for further lowering the noise are strong and more sophisticated technologies are required.
The object of the present invention is to provide an electrical steel sheet, for a low noise transformer, excellent in magnetostriction properties, capable of reducing high frequency components in a magnetostriction waveform and lowering noise effectively.
The embodiments of the present invention are as follows:
(1) A grain-oriented electrical steel sheet for a low noise transformer, having a film that imposes a tension of 0.5 MPa to 4.0 MPa on the steel sheet.
(2) A method for producing a grain-oriented electrical steel sheet for a low noise transformer, characterized by imposing a film tension of 0.5 MPa to 4.0 MPa on the surface by coating or a method corresponding thereto, without forming a glass film on the grain-oriented electrical steel sheet or, if a glass film is formed, after removing the glass film by an arbitrary method.
(3) A grain-oriented electrical steel sheet for a low noise transformer characterized by having a xcex19 of 1.5xc3x9710xe2x88x926 or less.
(4) A grain-oriented electrical steel sheet for a low noise transformer according to the item (1) or (3), characterized by having a sheet thickness of 0.27 mm or more.